Esophageal cardiac pulse monitoring apparatus and method

ABSTRACT

An esophageal cardiac pulse probe includes a lumen for insertion into the esophagus, the end of the lumen preferably being closed by a flexible diaphragm. Pressure variations imparted to the fluid within the lumen in response to sounds from the heart and the lungs are transmitted to an electrical transducer which produces an electrical signal proportional to the time-varying frequency and intensity of the pressure variations. This signal is selectively filtered to effectively eliminate signal components due to respiratory noise and audible heart sounds and the resulting signal is fed to an appropriate visual display apparatus. Direct acoustic cardiac sound monitoring is also achievable with an earpiece connected to the lumen. Several probe geometries and a method of cardiac pulse waveform monitoring are also disclosed.

This is a continuation of application Ser. No. 134,819, filed Mar. 28,1980, now U.S. Pat. No. 4,331,156.

TECHNICAL FIELD

The present invention relates in general to devices and methods formonitoring the cardiac pulse. More particularly, the invention concernsesophageal stethoscopes and devices and techniques for monitoring thecardiac pulse waveform using such stethoscopes.

BACKGROUND ART

Esophageal stethoscopes were first described nearly three decades agoand have been in clinical use during anesthesia since then to enable theanesthesiologist to obtain a rather direct acoustic measure of theheart's performance. Such acoustic esophageal stethoscopes typicallyhave comprised a tube or lumen which is inserted into the patient'sesophagus to a location at which pressure variations due to heart andrespiratory sounds are best transmitted to the interior of the lumen, bymeans such as a flexible diaphragm. The anesthesiologist is providedwith an earpiece connected to the lumen by a suitable conduit, so thatan acoustic indication of heart activity is provided. In some instancesrather than an acoustic earpiece, the stethoscope has been provided withan electro-mechanical transducer, such as a piezo-electrical device,which produces a signal proportional to pressure variations in thelumen, the signal then being provided directly to a speaker or headsetfollowing amplification. In either case, determination of the actualcardiac pulse waveform, which is monitored for various reasons duringsurgery, has not been attempted via the esophageal stethoscope. Rather,external pressure transducers applied to the carotid artery orelectro-optical transducers applied to the fingers have been relied onto determine cardiac pulse waveform. For many reasons, these externaldevices are either not useful on certain patients or are somewhatunreliable. Thus, a need has continued to exist for a simple, reliablecardiac pulse monitor which can be used on most patients while they areunder anesthesia.

DISCLOSURE OF THE INVENTION

A primary object of the invention is to provide an improved esophagealstethoscope which can be used on most patients to provide an indicationof cardiac pulse waveform.

Another object of the invention is to provide such a stethoscope whichalso will provide a direct, acoustic indication of heart sounds.

A further object of the invention is to provide such a stethoscope inwhich cardiac pulse waveform and acoustic indication of heart sounds aredetermined in isolated portions of the stethoscope probe.

Still another object of the invention is to provide such a stethoscopewhich produces an electrical signal proportional to cardiac pulsewaveform, and includes means for conveying the signal to existing signaldisplay devices such as oscilloscopes and strip chart recorders.

Yet another object of the invention is to provide such a stethoscopewhich produces an electrical signal proportional to cardiac waveform,the signal being free of high frequency components primarily due toheart noises and low frequency components primarily due to respiratorynoises. These objects of the invention are given only by way of example;thus, other desirable objectives and advantages inherently achieved bythe disclosed invention may occur to those skilled in the art.Nonetheless, the scope of the invention is to be limited only by theappended claims. One aspect of the invention concerns an improved methodof determining the cardiac pulse waveform by monitoring pressurevariations in the esophagus. Means, such as a lumen, are used to conveythese variations to an electro-mechanical transducer which produces anoutput signal proportional to the frequency and intensity of thepressure variations. By filtering out low frequency componentsattributable mainly to respiratory noises and higher frequencycomponents attributable mainly to heart noises, a sub-audible signal isobtained which is proportional to the cardiac pulse waveform. Thesub-audible signal proportional to the cardiac pulse waveform has beenfound to be in the range of 0.1 to 30 Hz.

The probe used to convey pressure variations to the transducer may beconfigured in several ways. A simple, open-ended lumen can be used;however, its reliability is uncertain since liquids in the esophagus mayblock the open end and prevent transmission of pressure variations upthe lumen to the transducer. Improved performance is achieved if thesingle lumen is provided with radially extending holes adjacent itslower end and a flexible, cuff-like or tubular diaphragm surrounding andspaced from the lumen in the vicinity of the holes. The diaphragm thenexpands upon internal application of fluid pressure to just touch thewalls of the esophagus. This probe is similar to that used in the pastfor simple acoustic monitoring. Direct acoustic monitoring of heartsounds also is achievable using an acoustic earpiece connected to thesingle lumen; however, an isolation diaphragm should be included in theearpiece to prevent leakage.

In the preferred probe according to the invention, two lumens are used,one located within the other. The outer lumen is relatively flexible andthe inner one is considerably smaller and relatively stiff. At theirlower ends, the two lumens are isolated from each other by a sealextending between them. The outer lumen is provided with radiallyextending holes below the location of the seal. A tubular diaphragmsimilar to that just mentioned surrounds and is spaced from the outerlumen in the vicinity of the holes, so that pressure variations in theesophagus are transmitted to the inner lumen and from there to thetransducer. Where direct acoustic monitoring is also desired, the sealbetween the lumens is located between the ends of that portion of theouter lumen in which the holes have been provided; and the diaphragm isprovided with a reduced diameter portion which engages the outer lumenin the vicinity of the seal. Thus, pressure variations in the esophagusare transmitted to both the inner lumen and the space between the innerand the outer lumens. The transducer again is connected to the innerlumen and an acoustic earpiece is connected to the outer lumen. In thisinstance, an isolation diaphragm is not needed in the earpiece. Althoughthe arrangement is not optimum, parallel single lumens may also be used,each having an associated diaphragm at its lower end, one beingconnected to the transducer and one being connected to an acousticearpiece.

The signal processing circuitry according to the invention is simple andreliable. The piezoelectric transducers which are preferred in theinvention have a certain inherent capacitance. When the output signal ofthe transducer is fed to a buffer amplifier having a suitably highimpedance, the resultant effect is to filter out certain low frequencycomponents. In accordance with the invention, components below about 0.1Hz are filtered out, since they are attributable primarily torespiratory noises, sometimes referred to as the respiration artifact.Applicants have found that by further filtering the amplifier output toeliminate components above approximately 30 Hz, a sub-audible signalproportional to cardiac pulse waveform is obtained. The amplifier outputmay also be filtered to provide an audible signal proportional to heartsounds. By gating the signal proportional to pulse waveform at apreselected point in the output from a conventional electrocardiogrammachine, synchronized display of the two signals may be achieved on anoscilloscope or a strip chart recorder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic, partially exploded view of an esophagealcardiac pulse monitoring apparatus according to the invention.

FIG. 2 shows an enlarged, sectional view of an esophageal probe used inthe apparatus shown in FIG. 1.

FIGS. 3, 4 and 5 show enlarged sectional views of alternate forms ofesophageal probes useful in the apparatus shown in FIG. 1.

FIG. 6 shows an adapter with an isolation diaphragm which is used toprevent leakage past the acoustic earpiece.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described with reference to the drawings, in whichlike reference numerals identify like elements of structure in each ofthe several figures.

Referring to FIGS. 1 and 2, the cardiac pulse monitoring apparatusaccording to the invention is seen to comprise an esophageal probe 10which is assembled from an outer tube or lumen 12 and an essentiallyconcentric inner lumen 14. Lumen 12 is preferably 4 mm to 8 mm in outerdiameter and 3 to 5 mm in inner diameter; whereas, lumen 14 ispreferably 2 to 3 mm in outer diameter and 1 to 2 mm in inner diameter.To optimize gain and frequency response, the internal volume of lumen 14preferably is in the range of 0.5 to 3 cc. For lumens made from thepreferred vinyl tubing, lumen 14 is comparatively stiffer than lumen 12,which makes lumen 14 less sensitive to pressure variations acting on itswalls. Concentric lumen 12 is more compliant to the shape of theesophagus, and also protects lumen 14 from some pressure variations thatcould pass through its walls, as well as undesirable contact withadjacent body elements or hardware. The lower end 16 of lumen 14 extendsthrough a seal block or fitting 18 which preferably is sealed to bothlumens by means such as a suitable glue. Lumen 12 extends past sealblock 18 to its lower end 20 and on either side of seal block 18 isprovided with a plurality of radially extending holes 22 about itscircumference and along a portion of its length. A tubular diaphragm orballoon 24 extends along and surrounds the portion of lumen 12 in whichholes 22 are located. To maintain a radial spacing between diaphragm 24and lumen 12, the diaphragm includes radially extending shoulderportions 26, 28 projecting inwardly at its opposite ends and a reduceddiameter portion 30 engaging lumen 12 in the region where seal block 18is located. A cuff 32 extends upward along lumen 12 from shoulderportion 28; and a cuff 34 and spherical end closure 36 extend downwardalong lumen 12 from shoulder portion 26. Diaphragm 24 is formed usingconventional techniques from thin vinyl having a wall thickness of about0.015 to 0.025 cm, and has a maximum diameter in the range of 5.8 to12.2 mm.

Cuff 32 preferably is glued to the exterior of lumen 12; however,reduced diameter portion 30 preferably engages lumen 12 with a simpletight fit, for purposes to be described subsequently.

As shown in FIG. 2, diaphragm 24 and seal block 18 cooperate with lumens12 and 14 to define a lower pressure responsive volume 38 and an upperpressure sensing volume 40, each responsive to pressure variations inthe esophagus and each isolated from the other. In one actualembodiment, volume 40 was approximately 2.54 cm in length and volume 38was approximately 1.91 cm in length. Volume 38 communicates with theinterior passage of lumen 14; and volume 40, with the annulus betweenthe two lumens. Lumen 14 is led out of probe 10 through a suitablefitting 42 which is inserted into the upper end of lumen 12. A three-waystop cock 44 is attached to the end of lumen 14 at one of the stopcock's ports and to electromechanical transducer 46 at another of itsports. The remaining port of stop cock 44 is adapted to receive thedelivery conduit of a simple syringe 48. Fitting 42 also connects theannulus between lumens 12 and 14 to an acoustic earpiece 50, via anexpansion bellows 52 and conduit 54. In use, syringe 48 is used toinject a small amount of air into lumen 14, such as approximately 3 cc,so that diaphragm 24 is certain to expand into light contact with thewall of the esophagus and yet to remain compliant.

If excess air is injected into lumen 14 or lumen 14 becomesoverpressurized for any reason, leakage from volume 38 into volume 40past reduced diameter portion 30 will relieve the overpressure. Tofacilitate this pressure relief, the tightness of portion 30 aroundlumen 12 may be adjusted as desired by appropriate selection of theinner diameter of diaphragm 24 at portion 30.

The probe is inserted into the patient's esophagus prior to inflation ofdiaphragm 24 where it surrounds volume 38. The depth of insertion shouldbe at least to the point where the aorta crosses the esophagus; however,experience has shown that better results are achieved when the lowerportion of diaphragm is positioned where the left ventricle of the heartis closest to the esophagus. Following inflation, pressure variationsare transmitted up lumen 14 to transducer 46, which preferably is apiezoelectric transducer such as the Model 1010C made by TransmedScientific of San Luis Obispo, Calif. Transducer 46 thus produces anelectrical signal on line 56 which is proportional to the varyingfrequency and intensity of the pressure variations in the esophagus.These signals are fed to the input of a high impedance buffer amplifier58 which is selected so that its input impedance is high enough, in viewof the inherent capacitance of the transducer, effectively to filter outcertain low frequency components of the signal, particularly those belowabout 0.1 Hz. Applicants have found that components below this level areattributable primarily to respiratory noises and may be ignored whendetermining cardiac pulse waveform. The output from amplifier 58 is fedin parallel to a low band pass filter 60 and a high band pass filter 62.Filter 60 is chosen to pass components of the signal below approximately30 Hz. The resultant sub-audible signal has components in the 0.1 to 30Hz range and has been found by applicants to be proportional to thedesired cardiac pulse waveform. This signal is fed to a conventionalD.C. restoration circuit 64 which may include an optional gating featurecontrolled by the occurrence of a pre-selected portion of the output ofa conventional electrocardiogram unit 66. The output signals from boththe esophageal stethoscope according to the invention and theelectrocardiogram unit 66 thus may be synchronized as desired prior tovisual presentation on a visual display unit 68, such as anoscilloscope, strip chart recorder or both. Preferably, the apparatusaccording to the invention is provided with a connector plug 70 forconveying its output signal to display unit 68, via an input socket alsointended for carotid artery pulse monitors or electro-optical pulsemonitors. Synchronization between the cardiac pulse wave form producedby the invention and the electrocardiogram waveform may be done invisual display unit 68, if desired.

The output from band pass filter 62 is limited to components in theaudible 70 to 200 Hz range and is passed to means such as aphonocardiogram 72 which provides an audio output of actual heartsounds, in the known manner.

FIG. 3 shows an alternate esophageal stethoscope probe 74 according tothe invention. Here, a pair of lumens 76, 78 are fed in parallel to thelower portion of the probe where they enter a third tubular section orlumen 80. Lumen 80 may also extend to the upper end of the probe tosimplify handling and protect lumens 76, 78. An upper seal block 82extends between lumens 76, 78 and the inner wall of lumen 80. Lumen 76extends just through seal block 82; whereas, lumen 78 extends on througha further seal block 84 located between the ends of lumen 80. Aplurality of radially extending holes 86 are provided in lumen 80 oneither side of seal block 84, the holes extending about thecircumference of lumen 80 and along the portion of its length below sealblock 82. A tubular diaphragm 24 surrounds the portion of lumen 80 inwhich the holes 86 are located, in the manner previously described withrespect to FIG. 2. In use, lumen 78 is connected to earpiece 50; andlumen 76, to transducer 46.

FIG. 4 shows another esophageal stethoscope probe 90 according to theinvention. In this case, only pressure responsive volume 38 is providedwhich is connected to lumen 14. Direct acoustic transmissions of heartsounds are not monitored with this probe since the annulus betweenlumens 12 and 14 does not communicate with tubular diaphragm 24.Otherwise, the probe is used just as in the case of the embodiments ofFIGS. 2 and 3.

FIG. 5 shows yet another esophageal probe 92 which may be used inaccordance with the invention. In this case, all pressure variations tobe monitored either electronically or acoustically are conveyed upwardfrom the esophagus through a single lumen 94. To prevent leakage of airthrough earpiece 50, an adapter 96 is inserted in conduit 54, whichincludes a hermetically sealed isolation diaphragm 98 as shown in FIG.6. Pressure variations in conduit 54 thus cause diaphragm 98 to vibrateand transmit the variations to the doctor's ear, without allowingdiaphragm 24 to depressurize.

COMMERCIAL APPLICABILITY

The invention is disclosed for use in esophageal stethoscopes in which agas is used as the pressure transmitting medium. Those skilled in theart will appreciate, however, that a liquid could be used to transmitpressure variations, particularly through lumen 14 to transducer 46,without departing from the scope of the invention.

Having described our invention in sufficient detail to enable thoseskilled in the art to make and use it, we claim:
 1. An improved doublelumen apparatus for use in an esophageal stethoscope, comprising:a firstlumen having a first end for insertion into the esophagus; a secondlumen surrounding said first lumen, said second lumen having a secondend for insertion into the esophagus and a plurality of radiallyextending holes spaced about its circumference in the region of saidfirst end; means associated with said first and second ends forisolating the interior of said first lumen from the interior of saidsecond lumen, said isolating means extending between said lumens at alocation spaced along said second lumen above said holes; and flexiblediaphragm means for transmitting pressure variations in the esophagusinto said first and second lumens, said diaphragm means comprising atubular portion extending along and surrrounding said second lumen atthe location of said holes, a pair of radially extending shoulderportions projecting inwardly from the opposite ends of said tubularportion, whereby said tubular portion is radially spaced from saidsecond lumen in the vicinity of said holes, and means for attaching saiddiaphragm to said second lumen.
 2. An improved double-lumen apparatusfor use in an esophageal stethoscope, comprising:a first lumen having afirst end for insertion into the esophagus; a second lumen having asecond end for insertion into the esophagus; means associated with saidfirst and second ends for isolating the interior of said first lumenfrom the interior of said second lumen, said isolating means comprisinga third lumen having a plurality of radially extending holes spacedabout its circumference and along a portion of its length, a first sealmeans positioned within said third lumen above said holes, said firstand second lumens passing sealingly through said first seal means, asecond seal means positioned within said third lumen, at a locationbetween the ends of said portion of said third lumen, said first lumenpassing sealingly through said second seal means and said second lumenterminating between said first and second seal means; and flexiblediaphragm means for transmitting pressure variations in the esophagusinto said first, second and third lumens, said diaphragm meanscomprising a tubular portion extending along and surrounding said thirdlumen at the location of said holes, said tubular portion having areduced diameter portion engaging said third lumen in the region of saidsecond seal means and a pair of radially extending shoulder portionsprojecting inwardly from the opposite ends of said tubular portion,whereby said tubular portion is radially spaced from said third lumenbetween said reduced diameter portion and said opposite ends, and meansfor attaching said diaphragm means to said third lumen.